Lesson Title: Constructing a Dichotomous Key and Exploring Its Relationship to
Evolutionary Patterns
NSF GK-12 Fellow: Tommy Detmer
Grade Level: 4th and 5th grade
Type of Lesson: STEM
Objectives: The objectives of this lab are to learn to use a dichotomous key, identify
variation in physical characteristics among closely and distantly related species, and
recognize the relationship between dichotomous keys and patterns of evolution. This
activity addresses the following Boulder Valley School District Essential Learnings for
fourth graders in describing that each organism has different structures that serve
different functions in growth, survival, and reproduction and for fifth graders in
explaining how nonliving components of ecosystems can affect living components.
Background Information: A dichotomous key is a set of couplets of mutually exclusive
physical characteristics that are used to identify organisms in a scientifically
reproducible way. Dichotomous keys are the most widely used method in the scientific
community for classification of organisms into different groups such as family, genus, or
species. Dichotomous keys are used by deciding between two exclusive characteristics
and following a path to the correct group or answer. They begin with general
characteristics and slowly lead to more specific traits and ultimately into a single
category identifying the organism (a more formal and scientific version of the kid’s game
20 questions to identify a person, place, or thing). Dichotomous keys are typically
organized around family trees. Family trees, also known as cladograms, are trees
diagraming the evolutionary relationships between different taxa. Historically, these
trees were based on physical characteristics, and this particular type of family tree is also
known as a phenogram. Although not always correct, as we now know from DNA
analysis, phenograms show relatively accurate trees. One of the goals of this activity is
to create a tree that will be incorrect based on physical characteristics in order to help the
students understand the benefits of using new techniques like DNA analysis.
This lab will encourage students to discover how variations in structures on animals can
be used to identify them and reveal information about their relationships to each other. In
this lab students will first learn to create a dichotomous key for a small group of different
organisms. The students will then, with your assistance, create a dichotomous key for all
the organisms in the class. Next, they will realize that the key they created can be used to
describe the evolutionary history of the organisms. Again, trees created using physical
University of Colorado, Project EXTREMES
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nearly, but not completely, correct. I have found that students love the drama of using a
method that has been used by scientists for centuries, but sometimes they come to an
incorrect conclusion. This nicely illustrates that scientists are still making major
discoveries about something as seemingly simple as the relationships between organisms.
Finally, when students recognize the inaccuracies of this method, you can explain that
DNA is now used to create relationship trees among organisms. DNA is much more
accurate because instead of using a few physical characteristics, scientists are able to use
thousands of base pairs. DNA is still very cost prohibitive and so as a result, as
previously described, dichotomous keys are the most commonly used method of
identifying species.
References: Vocabulary and general lesson structure were adapted from the following
websites:
http://www.mhhe.com/biosci/pae/zoology/cladogram/
http://www.biologyjunction.com/dichotomous_keying.htm
Lesson Vocabulary:
Family tree (cladogram if older students) – an upside down tree showing how closely
or distantly different organisms are related according to physical features or DNA
analysis.
Physical characteristic – Feature or features used to describe the way something looks
or feels
Dichotomous key – a tool for categorizing species or objects using logical choices.
DNA – Deoxyribonucleic acid is a molecule that encodes the genetic instructions for
creating all living organisms. It is made up of nucleotides given the letters ATCG;
our entire genetic code is made up of a four-chemical base structure.
Materials Required: This lab works best with the following small plastic animals but
will also work with pictures substituted for the plastic animals if they aren’t available
(both described here after as animal models). Both types of models, plastic and pictures,
can work interchangeably if one is not available. Animals in parentheses can substitute
for animals.
1. Octopus, five starfish, and squid (jellyfish, cuttlefish, oyster, mussel)
2. Butterfly, lady beetle, and cockroach (crab, lobster, bumble bee, spider)
3. Trout, grouper, whale (any form of fish for trout and grouper and dolphin or seal
for whale)
4. Frog, snake, lizard (toad, newt, dinosaur)
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6.
7.
8.
Eagle, duck, bat (any bird will substitute well for the eagle and duck)
Lion, tiger, bear (wolf, mountain lion, coyote)
Rabbit, deer, squirrel (buffalo, elk, giraffe)
Lemur, chimp, human (any tailed monkey will substitute for lemur and any
untailed monkey will substitute for chimp)
Preparation: (15 minutes)
Animal models should be separated into small groups before class.
Engagement: Tell the students that dichotomous keys are the most frequently used
method of identifying different organisms in the scientific community, and if they learn
how to use them correctly they can use them to identify almost any creature on Earth.
Show them animal models of a lion, a bear, and a snake and ask them which one is least
like the others. The students should answer the snake, which is a reptile. Ask the
students what physical characteristic of the mammals makes the reptile different. Have
the students fill out the first couplet together as a class for question #1. Do this again
with the second couplet. If they are struggling, it can be helpful to tell students that for
each couplet they should come up with a trait that some of the animals have and some
don’t (essentially yes and no traits). For example, for in the first question about, the
snake, the lion, and the bear, the snake has scales and the lion and the bear have no
scales.
Exploration: Hand out the animal models to the students in the previously described
groups. Have the students create a small family tree, like the one in question 1, for
question 2 using the animal
models for their group.
Next, allow the students to
discuss in their groups the
characteristics that all of the
animals in the tree they just
created share. One member
of each group should briefly
describe these to the class.
Create a dichotomous key,
with the help of the students,
on the board incorporating
all of the groups (see
example board). If the
students have trouble getting
started try to lead them by
asking which one group is most different and then the same question among the rest.
University of Colorado, Project EXTREMES
http://cires.colorado.edu/education/outreach/extremes/index.html Page 3 Explanation: Lead the students in a discussion on the relationships between different
species; the closer the species being compared are on the family tree the more likely they
are closely related evolutionarily. Hopefully, one or two of the students will point out the
errors of this cumulative tree, notably that the positions of the whale and the bat are
incorrect and that they should be positioned with the mammals and not with the fish and
birds. This may confuse some of the students. A good way to help students that are
confused is by telling them how different groups can evolve to have similar structures if
that structure is beneficial in a particular environment (such wings for bats and fins for
whales).
Elaboration, Extension: The students will have created a tree using only one set of
external physical characteristics. Have them fill out question 4 asking how they might
make a more accurate tree (comparing trees with more features, internal anatomy in
addition to external anatomy, and DNA).
Evaluation: For question five, have the students come up with a three couplet
dichotomous key including flowers, insects, and mammals.
Wrap-up: Ask the students to think about dichotomous keys and evolution on their way
home, could they create a key for things such as shoes, books, or computers?
University of Colorado, Project EXTREMES
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Name
Date
Question 1
Create a dichotomous key for the following organisms: a lion, a bear, and a
snake
Has
Does not have
Has
Does not have
Question 2
Create a dichotomous key for your groups of animals using physical
characteristics
Question 3
University of Colorado, Project EXTREMES
http://cires.colorado.edu/education/outreach/extremes/index.html Page 5 Create a dichotomous key of all the animal groups for the whole class
(complete this with the whole class)
University of Colorado, Project EXTREMES
http://cires.colorado.edu/education/outreach/extremes/index.html Page 6 Question 4
How could you make a more accurate family tree?
Question 5
Come up with a four couplet dichotomous key including a maple tree, a
flower, a bumble bee, and a polar bear
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